Flue gas purification plant

ABSTRACT

A plant for purifying the flue gases from a furnace, in particular from a rotary cement kiln, has at least one selective reduction catalyst for reducing nitrogen oxides present in the flue gas, optionally introduction of a reducing agent and a dust filter or precipitation system. The dust precipitation system is formed by at least one first and one second filter apparatus. The reduction catalyst is arranged between the first and second filter apparatuses.

The invention relates to a plant for purifying the flue gases from afurnace, including at least one selective reduction catalyst forreducing nitrogen oxides present in the flue gas, and/or at least onecatalyst for reducing carbon monoxide from, particularly odor-forming,hydrocarbons or for removing ammonia, as well as a dust separationmeans, and a method for purifying the flue gases from a furnace by theselective catalytic reduction of nitrogen oxides by a reducing agent anda reduction catalyst as well as by dust separation.

The removal of nitrogen oxides, or denitrification, is usually carriedout by reductive methods. In this respect, distinction is made betweenthe selective non-catalytic reduction (SNCR) and the selective catalyticreduction (SCR). These methods are known to degrade the nitrogen ornitric oxides (NO_(x)) contained in flue gases, i.e. nitrogen monoxideand nitrogen dioxide, by the addition of a reducing agent—in general,ammonia is used as a reducing agent—to elementary nitrogen and water,which will subsequently leave the combustion plant via the flue gasstack as environmentally harmless substances.

The selective non-catalytic reduction is usually carried out attemperatures between 900° C. and 1100° C., with the reducing agent beingdirectly fed into the furnace.

The selective catalytic reduction can be performed at distinctly lowertemperatures, since the catalyst significantly reduces the activationenergies required for the reducing reactions. This method, moreover,allows for a decrease of the reducing agent charge as compared to SNCRdenitrification, where the reducing agent is used in ahyperstoichiometric amount, since practically no side reactions willoccur.

The selective catalytic reduction differentiates between high-dustconfigurations and low-dust configurations. With high-dustconfigurations, the denitrification of the flue gases takes place priorto dedusting, which is why the catalyst is subject to elevated wear.This will, as a rule, result in reduced dwell times of the catalyst, andcall for expensive measures, e.g., the use of special catalysts withsuitable geometries of the flue gas channels extending through thecatalyst bed as is, for instance, described in DE 296 23 503 U1 or DE196 35 383 A1, or the use of catalysts that are capable of withstandinghigher mechanical stresses caused, for instance, by the periodicshaking-off of the dust loads from the catalyst particles.

With the low-dust concept, the selective catalytic reduction is providedfollowing the flue-gas desulfurization such that additional loads by SO₂and dust will be eliminated, the catalyst thus having an accordinglyextended dwell time. However, this involves the drawback of the fluegases only having temperatures of below 200° C. so as to requirereheating for the catalytic denitrification. An accordingly highadditional demand of energy will thus be required.

In the prior art, e.g. DE 196 12 240 A1 and DE 196 12 240 A2, solutionsin which the catalyst is added to the flue-gas stream in powdery formand separated on a bag filter together with the dust also have alreadybeen proposed. The reduction in those cases takes place during the flowof the flue gases toward the bag filter. The catalyst is subsequentlyperiodically blown off with the bag filter hot air while beingregenerated. However, this entails an additional load of dust on thefilter, thus calling for more frequent cleaning of the same.

It is the object of the present invention to provide a plant for theremoval of nitrogen oxides from flue gases according to the selectivecatalytic reduction (SCR) technology and a method therefor, whichcomprise reduced energy consumptions as compared to low-dust conceptsand increased catalyst dwell times as compared to high-dust plants.

This object of the invention is achieved by the initially defined plantin which the dust separation means is comprised of at least a first anda second filter device and the reduction catalyst is arranged betweenthe first and second filter devices, as well as by method for purifyingthe flue gases from a furnace, in which the flue gases are supplied to afirst dust separation prior to contacting the reduction catalyst, andthe fine-dust purification of the flue gases is effected after thereduction of the nitrogen oxides.

It was surprisingly found that, for the use of conventional catalystbeds which are not specifically formed for high-dust operation, it isnot necessary to dedust the flue gases at least nearly completely, butthat a pre-dedusting step or coarse-dust separation performed prior tothe catalytic reduction will do for the operation of the catalyst bedwithout substantially shortening the dwell time of the catalyst.

In this respect, it should be noted that “coarse-dust separation” and“fine-dust separation” do not necessarily refer to particle sizes, butrather to the dust load of the flue gas itself, i.e. the portion of dustparticles present in the flue gas.

The catalyst configuration for the removal of nitric oxides according tothe invention offers the advantage that the flue gases need not beadditionally heated for the denitrification reactions, but still haveinherently sufficient energy contents, i.e. sufficiently hightemperatures, to enable the operation of the catalyst. A reduction offuels is thus feasible as compared to low-dust plants.

The first filter device can be arranged immediately following thefurnace or a heat exchanger unit, viewed in the flow direction of theflue gases, such that the flue gases will enter the first filter unit ata very high temperature, whereby the temperature drop in this filterunit can be kept low—considered relatively—and the flue gases will leavethe filter device at a temperature promoting the reduction of thenitrogen oxides on the catalyst.

By high temperature, a temperature of at least 250° C. is understood.

Yet, it may also be envisaged to arrange a desulfurization plant betweenthe first filter device and the furnace, or a heat exchanger unitfollowing thereupon, viewed in the flow direction of the flue gases,such that the sulfur content will at least be proportionally reduced andthe risk of sulfur compounds depositing on the catalyst will be lowered.

In a particularly preferred manner, the first filter device is comprisedof an electric filter. This has the advantage that, on the one hand,this filter can be operated at a high temperature and, on the otherhand, this filter technology is already highly mature and electricfilters are available, anyway, for instance, in cement productionplants—dedusting formerly having been frequently performed with electricfilters, yet the majority of those electric filters having been replacedwith cloth filters due to tightened environmental regulations—and noadditional investment costs will occur.

It is, furthermore, possible that at least one raw material dryingapparatus or raw material dry-grinding apparatus is arranged between thefirst filter device and the second filter device, in particular betweenthe second filter device and the catalyst, viewed in the flow directionof the flue gases, such that the residual energy contents of the fluegases can be used for drying the raw materials which are, for instance,employed for the production of cement. In addition, it will be achievedthat the flue gases leaving the catalyst bed will even be further cooledprior to entering the second filter device, which is preferably formedby a cloth filter, said second filter device thus being subjected to areduced thermal stress even without applying additional cooling means.

The dust content of the flue gases in the first dust separation ispreferably reduced to a maximum dust content of 3 g/Nm³, in particular2.5 g/Nm³, for instance 1 g/Nm³, or a maximum dust content of 30 g/Nm³,respectively, if another pre-separation means, i.e. no electric filter,is used as said first filter device, since it has turned out that theefficiency of the plant can be enhanced by these maximum dust contentsof the flue gases.

It is, furthermore, possible to perform the first dust separation at atemperature of the flue gas of at least 250° C. or at most 450° C., forinstance at most 350° C., whereby, as already pointed out above, nospecial measures need to be taken to reduce the temperature drop in thefirst filter device, and the latter can hence be configured in a morecost-effective manner.

For a better understanding of the invention, the latter will beexplained in more detail by way of the following FIGURE.

In a schematically strongly simplified manner, FIG. 1 illustrates aplant according to the invention in the form of a block diagram.

To begin with, it should be noted that positional indications given inthe description, such as e.g. upstream, after or downstream, laterallyetc., refer to the FIGURE actually described and illustrated and anypositional change is to be analogously transferred to the new position.

FIG. 1 depicts a plant 1 for the production of cement clinker.

It has already been mentioned in the beginning that the nitrificationplant according to the invention is not limited to its use in the cementindustry, although this is the preferred variant embodiment. It is alsopossible to equip waste incineration plants, calorific power stations,etc. with the same.

The plant 1 comprises a furnace 2 in the form of a rotary kiln which isoperated by a firing device 3 so as to produce cement clinker from knownraw materials.

The flue gases leaving the furnace—arrow 4—are introduced into a heatexchanger unit 5, which in the present variant embodiment is designed asa cyclone heat exchanger comprising four cyclones, to utilize the energycontents of the flue gases for preheating the charged raw meal.

The flue gas leaving the heat exchanger unit 5—arrow 6—subsequentlyenters the gas purification plant. The gas purification plant comprisesa first filter device 7, a reduction catalyst 8 and a second filterdevice 9.

The first filter device 7 is configured as an electric filter. The fluegas entering the electric filter may optionally be pre-conditioned withwater in order to increase the effectiveness of the electric filter. Tothis end, a spray device 11 can be arranged in a supply line 10 leadingto the first filter device 7 to spray water into the same.

It is, furthermore, possible that the flue gas is diluted with withfresh air via a fresh-air duct 12 including a valve 13, so as to enablean increase in the effectiveness of coarse dedusting through theelectric filter. Alternately or additionally, a mixed gas can besupplied to the flue gas via a mixed-gas line 14, for instance a gasderived from the furnace 2, a so-called bypass gas, which can be drawnoff the furnace 2 in the region of the heat exchanger unit 5.

Appropriate conveying means 15, e.g. flue gas fans, can be arranged bothin the mixed-gas line 14 and in the supply line 10.

The dust content of the flue gas or crude gas is reduced by the firstfilter device 7 from 200 g/Nm³ to 300 g/Nm³ or 60 g/Nm³ to 70 g/Nm³,respectively, to a maximum value of 3 g/Nm³ and, preferably, 1 g/Nm³. Itis also possible to reduce the dust content only to a maximum of 30g/Nm³, if no electric filter but another pre-separation means is used assaid first filter device 7.

The first filter device 7 can be provided with a heat insulationsuitable for such high temperatures so as to lower the decrease of theflue gas temperature.

After this, the flue gas enters the reduction catalyst 8, where thedenitrification, i.e. the reaction of nitric oxides to nitrogen andwater, takes place according to known reactions. In doing so, it isprovided that a reducing agent is fed to the pre-purified flue gas,using a reducing-agent feeder 16. As a rule, the reducing agent iscomprised of ammonia, as is known from the prior art. Yet, the use ofcompounds containing ammonia or reducing agents releasing ammonia atelevated temperatures may also envisaged.

The reducing-agent feeder 16 may also be omitted if excess ammonia ispresent in the flue gas of the plant 1, and, if there is insufficientammonia, it is also possible to supplement the missing portion via thereducing-agent feeder 16.

The catalyst may, for instance, be comprised of titanium dioxide orvanadium pentoxide or titanium oxide as the carrier and vanadiumpentoxide as the active mass, optionally supplemented with tungstenoxide or mixed with other metal oxides. In principle, these catalystsare known from the prior art so that any further discussion as to theirgeometries or pore structures can be obviated.

The supply of reducing agent again is, for instance, effected throughspraying nozzles.

The reducing agent itself can be admixed to the flue gas up-stream ofthe catalyst, yet the reducing agent is preferably fed into or onto thecatalyst bed.

The formation of the catalyst bed per se is also known from the priorart such that in this respect it is referred to the pertinentliterature. It is, in particular, possible to provide the reductioncatalyst on several superimposed levels, through which the flue gas willsuccessively flow.

Via a line 17, the denitrified flue gas—it should be noted here that bydenitrified flue gas a flue gas is meant which, in terms of NO_(x),complies with the exhaust emission standards, e.g. the Austrian exhaustemission standards—reaches the second filter device 9. This secondfilter device 9 is configured as a bag filter comprising filter clothsor filter bags. Such bag filters are already known and used in thecement industry such that any further discussion can be obviated. By theaid of these filter cloths, the dust content of the flue gas is at leastreduced to values complying with the exhaust emission standards.

If required, a spray cooler 18 can be arranged upstream of the secondfilter device 9 to cool the flue gas prior to entering the second filterdevice 9 to a temperature, for instance a maximum temperature of 250°C., at which the thermal stress exerted on the filter cloth by the fluegas will be reduced.

After the second filter device 9, the thus purified flue gases leave theplant 1 into the air via a stack 19.

To this end, a conveying means 15 may again be arranged between thestack 19 and the second filter device 9.

The residual energy contents of the flue gases leaving the reductioncatalyst 8 are, however, preferably used to dry the raw materials usedfor cement production. To this end, two drying mills 20 are illustratedin FIG. 1, which are arranged between the reduction catalyst 8 and thesecond filter device 9, viewed in the flow direction of the flue gases.The two drying mills 20 are, in particular, arranged in parallel suchthat the denitrified flue gases will flow through the same eithersimultaneously or alternately. Valves 21-24 are illustrated in FIG. 1for the respective switching of the flow paths of the flue gases. It is,furthermore, shown that it is also possible to arrange the drying mills20 in parallel with the direct introduction of the flue gases via line17 into the second filter device 9, to which end a valve 25 is againarranged in line 17 in order to enable switching between the flowdirections via line 17 or via at least one of the drying mills 20.

The drying mills 20 themselves are configured according to the priorart.

Another option is to supply fresh air via a fresh-air duct 26 to atleast one of the drying mills 20, a respective fresh-air valve 27 beingarranged in the fresh-air duct 26 also in this case.

It is, furthermore, possible to admix at least a portion of the fluegases that have left the drying mills 20 to the flue gases supplied fromthe reduction catalyst 8 to the drying mills 20 so as to further exploitthe energy contained in these flue gases as circulating air via acirculating-air duct 28 including a circulating-air valve 29.

With this so-called “mill operation”, the denitrified flue gas willreach the second filter device 9 at a temperature of about 150° C.

In the context of the invention it is also possible to use more than twofilter devices 7, 9 and to arrange several reduction catalysts 8 inseries or in parallel. Moreover, more than two, or only one of the,drying mills 20 can be operated in the sense of the invention.

Although not illustrated, there is the option, as already pointed outabove, to arrange a desulfurization plant between the furnace 2 and thefirst filter device 7, or between the heat exchanger unit 5 and thefirst filter device 7, which may correspond to the prior art, in orderto reduce the sulfur content, i.e. the content of SO₂, in the flue gasat least proportionally.

During the operation of the plant 1, the following measured values wereobtained by appropriate sensors on the respective locations:

Dust content of the flue gas leaving the furnace 2 and/or the heatexchanger 5: 60 to 120 g/Nm³

Dust content of the flue gas leaving the electric filter: max. 3 g/Nm³

Content of NO_(x) after the reduction catalyst: less than 100 mg NO₂/Nm³

Temperature of the flue gas after the electric filter: 300° C. to 340°C.

Temperature of the flue gas following the reduction catalyst 8: 280° C.to 320° C.

Temperature of the flue gas at the entry into the bag filter: max. 250°C.

Dust content of the flue gas at the entry into the bag filter: less than3 g/Nm³ with direct introduction and about 100 g/Nm³ with “milloperation”, respectively

Dust content of the flue gas when leaving the bag filter: 10 mg/Nm³

As is, in particular, apparent from the above measurements, the dustcontent of the flue gas is practically not reduced in the reductioncatalyst 8. If, however, a dust precipitate does occur in the reductioncatalyst 8, this can be periodically cleaned, e.g. by purging withcompressed air.

According to a variant embodiment of the invention, it is possible touse, as an alternative or addition to the nitric oxides, also the fluegas of carbon monoxide from, particularly odor-forming, hydrocarbonsand/or the removal of ammonia from flue gases from combustion furnaces,particularly from plant 1. To this end, a separate catalyst can, ifrequired, be arranged upstream or downstream of the denitrificationcatalyst, comprising, for instance, titanium-vanadium compounds whichmay be supplemented with palladium and/or platinum. Such catalysts areknown from the prior art so that reference is made to the pertinentliterature. To this end, the reduction catalyst 8 can be configured as alayered catalyst including several beds for the individual catalysts, orseveral catalysts can be separately arranged in the plant 1 or arespective flue gas purification plant, for instance, one behind theother in separate containers, viewed in the flow direction of the fluegases.

For good order's sake, it should finally be pointed out that, for abetter understanding of the structure of plant 1, the latter and itscomponents have partially been illustrated out of scale and/or enlargedand/or downscaled.

LIST OF REFERENCE NUMERALS

-   1 plant-   2 furnace-   3 firing device-   4 arrow-   5 heat exchanger unit-   6 arrow-   7 filter device-   8 reduction catalyst-   9 filter device-   10 supply line-   11 spray device-   12 fresh-air duct-   13 valve-   14 mixed-gas device-   15 conveying means-   16 reducing agent-   17 line-   18 spray cooler-   19 stack-   20 drying mill-   21 valve-   22 valve-   23 valve-   24 valve-   25 valve-   26 fresh-air duct-   27 fresh-air valve-   28 circulating-air duct-   29 circulating-air valve

1-8. (canceled)
 9. A plant for purifying flue gas from a furnace, theplant comprising: at least one selective reduction catalytic converterfor reducing nitrogen oxides present in the flue gas and/or a catalyticconverter for reducing carbon monoxide, hydrocarbons, and/or ammonia; anoptional reducing-agent feed device; and a dust separation device formedof at least one first filter device and at least one second filterdevice, and having said selective reduction catalytic converter disposedbetween said first and second filter devices.
 10. The plant according toclaim 9, wherein the furnace is a cement rotary kiln.
 11. The plantaccording to claim 9, wherein said first filter device is disposedsubstantially immediately following the furnace or a heat exchangerunit, in a flow direction of the flue gas.
 12. The plant according toclaim 9, which further comprises a desulfurization plant disposedbetween said first filter device and the furnace, in a flow direction ofthe flue gas.
 13. The plant according to claim 9, which furthercomprises a heat exchanger unit connected immediately following thefurnace in a flow direction of the flue gas, and a desulfurization plantdisposed between said heat exchanger unit and said first filter devicein the flow direction of the flue gas.
 14. The plant according to claim9, wherein said first filter device is an electric filter.
 15. The plantaccording to claim 9, which further comprises at least one raw materialdrying apparatus connected between said catalytic device and said secondfilter device, in a flow direction of the flue gas.
 16. A method ofpurifying flue gas generated in a furnace, which comprises: providingthe plant according to claim 9 connected to the furnace; guiding theflue gas from the furnace to the first filter device of the dustseparation device; subsequently conducting the flue gas into contactwith a selective reduction catalytic converter for selective catalyticreduction of nitrogen oxides by a reducing agent and/or at least onecatalytic converter for reducing carbon monoxide, hydrocarbons, and/orammonia; and subsequently subjecting the flue gas to a fine-dustpurification by filtering dust from the flue gas in the second filterdevice of the dust separation device.
 17. The method according to claim16, wherein the flue gas is generated in a cement rotary kiln.
 18. Themethod according to claim 16, which comprises reducing a dust content ofthe flue gas in the first filter device to a maximum dust content of 30g/m³N before the flue gas is conducted to the catalytic device.
 19. Themethod according to claim 18, which comprises reducing a dust content ofthe flue gas in the first filter device to a maximum dust content of 3g/m³N.
 20. The method according to claim 19, which comprises reducing adust content of the flue gas in the first filter device to a maximumdust content of 2.5 g/m³N.
 21. The method according to claim 16, whichcomprises carrying out a first dust separation in the first filterdevice at a temperature of the flue gas of at least 250° C.
 22. A methodof purifying flue gas generated in a furnace, which comprises: guidingthe flue gas from the furnace to a first filter device of a dustseparation device and subjecting the flue gas to a first dustseparation; subsequently conducting the flue gas for contact with aselective reduction catalytic converter for selective catalyticreduction of nitrogen oxides with a reducing agent and/or a catalyticconverter for the reduction of carbon monoxide, hydrocarbons, and/orammonia; and subsequently, following the catalytic reduction, subjectingthe flue gas to a fine-dust purification in a second filter device ofthe dust separation device.